1. Field of the Invention
The present invention relates to sensing the position of an object in a plane and, more specifically, sensing the position of a stylus on a surface using a single camera and two mirrors.
2. Related Art
Users of pen-based computers generally input dam into a computer by writing with a special electronic stylus directly on the computer's liquid-crystal screen, rather than by typing on a keyboard. Program choices can be made by checking boxes on the screen, as if one is filling out an electronic form. Documents can be edited by making movements with the stylus on the screen, such as crossing out a word. Diagrams can be drawn directly on the screen.
When designing a pen-based computer system a significant consideration is determining the position of the pen or stylus. Many different techniques have developed to detect the stylus position. However, significant drawbacks still exist. The sensing (detecting) technology must be lightweight, consume little electronic power, be durable and withstand electromagnetic interference from other parts of the computer.
Moreover, capturing handwriting requires the system to be accurate to within, for example, five thousandths of an inch. This level of accuracy is far more difficult to achieve than that required in other applications such as determining whether a finger is pointing to the "deposit" box or the "withdrawal" box on the touch screen of an automated teller machine. If the pen-sensing system makes even small errors, the computer might not be able to make fine distinctions, such as discriminating the letter "Z" from the number "2," no matter how proficient the computer's handwriting-recognition software is. Without a highly precise mechanism for sensing the position of the stylus these computer systems will not reliably and accurately function.
In addition to reliability and accuracy, the position sensing mechanism must be cost effective, durable and, easy to use. Some technologies, for instance, require the stylus to be connected to the computer by a wire, while others allow for a stylus to be detached. In general, it is desirable for a stylus to be detachable from the computer and the sensing device in order to improve the system's flexibility. However, the cost of manufacturing the stylus should not be excessive since a detachable stylus may be easily misplaced.
In addition, some previous systems require the stylus to touch the screen in order to be detected. Other systems can detect the stylus when it is merely near the screen. It is desirable to have the ability to sense the stylus before it touches the screen in order to help the computer system provide instant feedback.
There are three basic pen-sensing technologies. One is a resistive coating technique in which the front of a screen is coated with indium-tin oxide, a transparent material that conducts electricity. The voltage in this coating is made to vary gradually from the top to the bottom of the screen and from the left to the right of the screen. When the stylus contacts the coating, an electrical current flows through the stylus and is transmitted to the computer via a wire. Since the voltage varies across the screen, the amount of electricity flowing through the stylus will vary depending on where it touches the screen. This technology is inexpensive. However, this technology requires the stylus to be wired to the computer and to make contact with the screen in order for the stylus position to be detected. In addition, since the coating is on the front of the screen, it is subject to wear.
A second approach uses electromagnetic waves, essentially a faint radio transmission from the stylus to the computer or vice versa. A grid of wires behind the screen emits a signal which causes a circuit in the stylus, which is tuned to the signal frequency, to resonate and transmit a signal of its own. The computer then stops transmitting and listens for the signal from the stylus. The position of the stylus is determined by the position of the wires in the grid which detect the signal emanating from the circuit in the stylus.
This second approach can sense the stylus location before it touches the screen. In addition, the stylus need not be connected to the computer by a wire and may be an unpowered, passive circuit. However, this system is expensive, with a stylus costing $80 or more. In addition, the grid of wires behind the screen make it more difficult for screens to be backlit. Therefore, screens using this system are typically more difficult to read.
A third approach is similar to the electromagnetic one, i.e., the second approach, but senses by electrostatic coupling rather than electromagnetic coupling. This system can also detect the stylus without requiring the stylus to contact the screen. However, the stylus must be active and therefore must either be wired to the computer or contain a battery. Styluses using this approach are less costly than those in the second approach, but they remain relatively expensive.
As is seen from the above discussion, in all three of the above approaches the drawing surface must be specially designed in order to determine the stylus location. That is, in the first approach, the drawing surface is coated with an indium-tin oxide. In the second and third approaches, a grid of wires must be positioned beneath the drawing surface.
What is needed is a system for accurately sensing the location of a stylus in a plane. The stylus should not require connection to the computer or to a drawing surface. The sensor should be compact while not requiring a special surface coating or an embedded wire grid. The system should be able to sense the position of the stylus before the stylus physically contacts an object such as a drawing surface. In fact, a drawing surface should not be required to determine the stylus position. In addition, the system should be durable and inexpensive to build and maintain.